The present invention relates to a method for purifying an alkali metal halide brine used in the electrolytic production of high purity alkali metal hydroxide solutions and more particularly to an improved process for removing chlorate ions therefrom. The alkali metal chloride brines used in the present invention are produced along in halide utilizing electrolytic cells by the passage of an electric current through said alkali metal halide brine. Electrolytic cells are commonly employed commercially for the conversion of alkali metal halide into alkali metal hydroxide and halide, fall into one of three general types--diaphragm, mercury and membrane cells.
Diaphragm cells utilize one or more diaphragms permeable to the flow of electrolyte solution but impervious to the flow of gas bubbles. The diaphragm separates the cell into two or more compartments. Further imposition of a decomposing current, halide gases, given off at the anode, and hydrogen gas along with an alkali metal hydroxide are formed in the cathode. Although the diaphragm cell achieves relatively high production per unit floor space, at low energy requirement and at generally high current efficiency, the alkali metal hydroxide product, or cell liquor, from the catholyte compartment is both dilute and impure. The product may typically contain about 12% by weight of alkali metal hydroxide along with about 12% by weight of the original, unreacted alkali metal chloride. In order to obtain a commercial or salable product, the cell liquor must be concentrated and purified. Generally, this is accomplished by evaporation. Typically, the product from the evaporator is about 50% by weight alkali metal hydroxide containing about 1% by weight alkali metal chloride.
Mercury cells typically utilize a moving or flowing bed of mercury as the cathode and produce an alkali metal amalgam from the mercury cathode. Halide gas is produced at the anode. The amalgam is withdrawn from the cell and treated with water to produce a concentrated high purity alkali metal hydroxide solution. Although mercury cell installations have many disadvantages including a high initial capital investment, undesirable ratio of floor space per unit of product and negative ecological considerations, the purity of the alkali metal hydroxide product is an inducement to its continued use. Typically, the alkali metal hydroxide product contains less than about 0.05% by weight of contaminating foreign ions.
Membrane cells utilize one or more membranes or barriers separating the catholyte and anolyte compartments in the cell. These membranes are permselective; that is, they are generally permeable to either anions or cations. Generally, the permselective membranes utilized are cationically permselective. In membrane cells employing a single membrane, the membrane may be porous or non-porous. The membrane cells employing two or more membranes, porous membranes are usually utilized closest to the anode and non-porous membranes are usually utilized closest to the cathode. The catholyte product of the membrane cell is a relatively high purity alkali metal hydroxide. Catholyte cell liquor from a membrane cell is purer and has a higher caustic concentration than the product of the diaphragm cell.
It has been the objective, but frequently not the result, for diaphragm and membrane cells to produce "rayon grade" alkali metal hydroxide, that is, a product having a contamination of less than about 0.5% of the original salt. Diaphragm cells have not been able to produce such a product directly, because anions of the original salt freely migrate into the catholyte compartment of the cell. Membranes cells do have the capability to produce such a high quality alkali metal hydroxide product. However, one problem encountered in the operation of such cells is the production of chlorate in the anolyte compartment which will not readily pass through a cation, permselective membrane. Accordingly, chlorates concentrate in the anolyte, and after brief period of operation, may reach objectionable concentration levels. While chlorates are not known to cause rapid deterioration of membrane or anode structures, high concentrations thereof do tend to reduce the solubility of the salt resulting in decreased efficiencies, possible salt precipitation and potentially adverse chlorate concentrations in the caustic product.
In the past, removal of chlorate from diaphragm cell liquor has been handled in a number of ways. For example, Johnson, in U.S. Pat. No. 2,790,707, teaches removal of chlorates and chlorides from diaphragm cell liquor by formation of iron salts by adding ferrous sulfate. Osborne, in U.S. Pat. No. 2,823,177, teaches the prevention of chlorate formation during electrolysis of alkali metal chloride in diaphragm cells by destruction of hypochlorite through distribution of catalytic amounts of nickel or cobalt in the diaphragm. It is noteworthy that considerable effort has been expended in chlorate removal from catholyte cell liquor, a highly alkaline medium. In such a solution, chlorate ion is quite stable and therefore tends to persist in the cell effluent and to pass on through to the evaporators in which the caustic alkalis are concentrated. Practically, all of the chlorate survives this evaporation and remains in the final product where it constitutes a highly objectionable contaminant, especially to the rayon industry.
The problem of lowering chlorates in diaphragm cells has been attacked at two main points:
(a) the chlorates having been formed, can be reduced in the further processing of the caustic alkali and by special treatments; or PA1 (b) production of chlorates during electrolysis can be lowered by adding a reagent to the brine feed which reacts preferentially with the back migrating hydroxyl ions from the cathode compartment of the cell making their way through the diaphragm into the anolyte compartment, and by such a reaction, prevents the formation of some of the hypochlorites and thus additionally preventing these hypochlorites from further reacting to form chlorates. Reagents such as hydrochloric acid or sulfur in an oxidizable form, such as sodium tetrasulfide, have been used to attack this problem.
In membrane cell operation, it is conventional to recycle spent brine from the anolyte compartment for resaturation. Satisfactory operation can be achieved so long as the chlorate concentration in the anolyte brine stream is kept below about 1.0% (i.e., about 10 g/l). In modern cells, the chlorate concentration buildup during the normal residence time of the anolyte brine solution therein is about 0.1% per pass. Thus, if the initial chlorate content in the anolyte brine is acceptable, it is not necessary to remove all the chlorate present but only enough to remove the additional chlorate formed in the cell during this residence time to keep the brine within usable limits. In the past, removal of chlorate sufficient to keep the brine satisfactory has been accomplished by purging a portion of the depleted brine and adding fresh brine as makeup. In many facilities, the purged chlorate containing brine is often used as feedstock in a separate chlorate cell.
More recently, Lai et al. in U.S. Pat. No. 4,169,773 have shown that chlorate concentrations in the circulating brine stream are significantly reduced by reacting a portion of said stream prior to dechlorination, with a strong acid such as HCl to produce additional chlorine, water and salt. In this procedure, substantially all the chlorate therein is removed therefrom, so that when said depleted portion is added back to the main stream, the average chlorate value is within acceptable limits. However, the system used by Lai et al. calls for a separate dechlorination subsystem for the treated brine which adds both to the complexity and costs for chlorate removal. What is needed is a simpler, less expensive procedure for chlorate removal for recirculating brine streams used in membrane cells. As shown by Dotson in "Kinetics and Mechanism for the Thermal Decomposition of Chlorate Ions in Brine Acidified with Hydrochloric Acid", J. Appl. Chem. Biotechnol., 1975, 25, 461-464, chlorate removal rate is a function of the chloride ion content and the higher this value, the more efficient is the process for chlorate removal.